1. Field of the Invention
The present invention relates to sludge solids beneficiated for use as fertilizer, and possessing improved resistance to combustion and corrosive effect. The invention also includes methods for its production.
2. Description of the Related Art
The disposal of sludges discharged from large-scale wastewater treatment plants is a serious and growing problem. In 1990, the United States Environmental Protection Agency indicated that a family of four discharged 300 to 400 gallons of wastewater per day. From this wastewater, publicly owned treatment works generated approximately 7.7 million dry metric tons of sludge annually or about 64 dry pounds of sludge for every individual in the United States. The agency indicated that these figures would double by the year 2000.
The definitions of xe2x80x9csewage sludgexe2x80x9d and xe2x80x9csludgexe2x80x9d given by Title 40 of the Code of Federal Regulations, Part 257.2 are hereby incorporated herein as follows:
xe2x80x9cSewage sludge means solid, semi-solid, or liquid residue generated during the treatment of domestic sewage in a treatment works. Sewage sludge includes, but is not limited to, domestic septage; scum or solid removed in primary, secondary or advanced wastewater treatment processes; and a material derived from sewage sludge. Sewage sludge does not include ash generated during the firing of sewage sludge in a sewage sludge incinerator or grit and screenings generated during preliminary treatment of domestic sewage in a treatment works.
Sludge means solid, semi-solid, or liquid waste generated from municipal, commercial, or industrial wastewater treatment plant, water supply treatment plant, or air pollution control facility or any other such waste having similar characteristics and effectxe2x80x9d.
There are several types of sludges that can be produced by sewage or wastewater treatment. These include primary sludge, waste activated sludge, pasteurized sludge, heat-treated sludge, and aerobically or anaerobically digested sludge. The sludges may be municipal or industrial.
Most commonly, sludges are dewatered to the best extent possible by chemical and mechanical means. The water content of sewage sludges is still very high. Typical sludges coming out of a gravity clarifier may have a dry solids content of 2% or less. After anaerobic digestion, the solids content can be about 10%. Cationic water-soluble polymers have been found useful for causing further separation between the solids and the water that is chemically and physically bound. Filtration or centrifugation of cationic polymer treated sludge typically yields a paste-like sludge cake containing about 20% solids
Drying of sewage sludge has been practiced for many years in both the United States and Europe. Sludge drying in the United States prior to about 1965 was undertaken to reduce transportation costs and in pursuit of various disposal options. In some plants, the sludge was dried in powder form and the fine particles were consumed in the combustion chamber of an incinerator or boiler In the late 1960""s, two municipalities, Houston and Milwaukee, began to market a pelletized or granulated dried sludge for use as a soil amendment and/or fertilizer. Several more plants for manufacture of dried pelletized sludge were built in the 1980""s and 1990""s; especially after ocean dumping of sludge by coastal cities was eliminated. Drying and conversion to a pelletized fertilizer was the best option for these metropolitan areas where landfills and land for disposal were limited. However, the investment required for a sludge drying facility is large. A typical unit costs about $150 million for equipment alone.
The most common type of sludge dried and pelletized is anaerobically digested municipal sewage. Anaerobic digestion, as the name suggests, involves treatment by facultative bacteria under anaerobic conditions to decompose the organic matter in the sludge. After a prescribed time and temperature, a sludge relatively free of putrifiable organic matter and pathogens is obtained. Municipal anaerobically digested sewage sludge is therefore preferred for agricultural purposes.
However, dry sewage sludge has several disadvantages for agricultural use. It has low fertilization value, typically having nitrogen content of only about 2-5%. Freight and application costs per unit of nitrogen are high. It often has a disagreeable odor, particularly when moist. It has low density and when blended with other commercial fertilizer materials, it may segregate into piles or may not spread on the field uniformly with other more dense ingredients. Bacterial action may continue and under storage conditions sludge temperature may rise to the point of autoignition. Hence, except for special markets that value its organic content for soil amendment or filler in blended fertilizer, there is little demand for the product. In most cases municipalities must pay freight charges, or must offer other incentives for commercial growers to use the material. However, this is frequently still more economical than alternative disposal schemes.
The market value of fertilizers is principally based on their nitrogen content. A need exists for a practical and economic method for increasing the nitrogen content of sewage sludge to a level approaching that of commercial mineral fertilizers, i.e. 10-20%. Freight costs and the cost of application per unit of nitrogen would then be much lower. Overall value and demand would increase. Moreover, sludge has an advantage in that its nitrogen is of the slow release type. The nitrogen is part of organic molecules and hence is available to growing plants only when the molecule is broken down. This is very desirable since it provides nitrogen to the plant all through its growing cycle. Manufactured slow release nitrogen fertilizers have a price nearly 10 times that of ordinary mineral nitrogen fertilizers. Conceivably, municipalities would enjoy a credit rather than an expense in disposing of their dried sludge product if the total nitrogen content can be increased and the tendency for autoignition reduced or eliminated.
Prior art attempts have been made to reach some of these objectives. U.S. Pat. Nos. 3,942,970, 3,655,395, 3,939,280, 4,304,588 and 4,519,831 describe processes for converting sewage sludge to fertilizer. In each of these processes a urea-formaldehyde condensation product is formed in situ with the sludge. However, the processes require the handling of formaldehyde: a highly toxic lachrymator and cancer suspect agent.
French Patent 2,757,504 describes the blending of mineral fertilizers with organic sludge. The mixture is heated to a temperature between 200xc2x0 C. and 380xc2x0 C. Japanese Patent 58032638 describes a process where sludge is treated with sulfuric and nitric acids or sulfuric and phosphoric acids and ammonia under elevated pressure of about 3 atmospheres. These prior art processes require costly process equipment and/or special conditions not readily incorporated in existing sewage treatment facilities.
The simplest method of increasing the nitrogen in sludge would be to add commercial nitrogen fertilizer materials to the wet sludge prior to drying and pelletizing. There are only a few high-nitrogen fertilizer materials that are economic for use in agriculture: ammonia (82 wt. % N), urea (37 wt. % N), and ammonium nitrate (35 wt. % N). Ammonia has high volatility and is subject to strict regulation of discharges to the atmosphere. Urea is a solid that adsorbs moisture quite readily and makes the sludge more difficult to dry. It is also highly susceptible to breakdown to ammonia by the microbes and enzymes in sludge, resulting in nitrogen loss and an odor problem. Ammonium nitrate is a strong oxidizer and creates a potential explosion problem. All of these fertilizers have high nitrogen content: but are unsuitable for combining with sludge.
Another possible candidate that has been unsuccessfully tested by the industry as an additive to sludge is ammonium sulfate. Although ammonium sulfate has lower nitrogen content (21 wt % N) than the materials discussed above, it has a price per unit of nitrogen comparable to that of the other commercial fertilizers. It is also relatively inert to the microbes and enzymes in sludge.
Unfortunately, it has been found in full-scale plant trials that a problem occurs during the drying of a mixture of ammonium sulfate and sludge. Title 40 of the Code of Federal Regulations, Part 503, Appendix B specifies that the temperature of the sewage sludge particles must exceed 80xc2x0 C. or the wet bulb temperature of the gas in contact with the sewage sludge must leave the dryer at a temperature exceeding 80xc2x0 C. However, when drying a mixture of ammonium sulfate and sludge, a sudden release of ammonia vapors occurs at about 60xc2x0 C. overwhelming the air pollution control system. Several attempts at addition of ammonium sulfate to sewage sludge in several different plants over several years have foundered on this problem. The discharge of ammonia to the atmosphere is environmentally intolerable. Consequently, ammonium sulfate addition to sewage sludge has not been successful to date.
European Patent 0 143 392 B1 describes a process in which an undigested liquid sludge is mixed with salts such as ammonium sulfate at a concentration of 17-47 wt. % at a pH of 2-6 for a period of 3 to 12 hours followed by disposal. The patent teaches away from drying this mixture. Japanese Patent 9110570 A2 describes the treatment of sewage sludge with an acidic solution followed by drying to reduce ammonia evolution and to retain the effective nitrogen. Honda et al., describe the use of dilute (0.3 Normal) aqueous solutions of HCl, H2SO4, and wood vinegar as ammonia binders (xe2x80x9cGranulation of Compost From Sewage Sludge. V. Reduction of Ammonia Emission From Drying Processxe2x80x9d, Hokkaidoritsu Kogyo Shikenjo Hokoku, 287, 85-89 (1988)). Neither of these latter references discloses the use of acids with ammonium sulfate additions and neither reference discusses the issue of corrosion of steel process equipment under acid conditions.
It will be seen that a long standing need exists for practical means of increasing the economic value of sewage sludge through increasing its nitrogen content, and decreasing its flammability while preventing the release of ammonia during drying and preventing corrosion of the process equipment. The present invention meets those needs in present sewage treatment facilities.
The present invention is a beneficiated sludge solids composition of increased nitrogen and phosphorus content, and increased economic value comprising digested municipal sewage sludge, ammonium sulfate, mineral acid, and phosphate salt. Preferably, the beneficiated sludge solids composition comprises digested municipal sewage sludge, ammonium sulfate and superphosphoric acid. The method of the invention comprises mixing digested municipal sewage sludge with ammonium sulfate, mineral acid and a phosphate salt followed by drying. Preferably, the method of the invention comprises mixing digested municipal sewage sludge with ammonium sulfate and superphosphoric acid followed by drying. In other embodiments, the method of the invention additionally comprises the step of granulating the beneficiated composition. The method of the invention avoids release of noxious gases, retards corrosion of the treatment equipment, and is practicable in current sewage treatment facilities.